Troubleshooting Routes Flapping Problem in RIP

Running RIP in a complex environment can sometimes cause flapping of routes. Route flapping refers to routes coming into and going out of the routing table. To check whether the routes are indeed flapping, check the routing table and look at the age of the routes. If the ages are constantly getting reset to 00:00:00, this means that the routes are flapping. Several reasons exist for this condition. This section discusses one of the common reasons ‚ packet loss because the packet is dropping on the sender's or receiver's interface. The example in this section considers Frame Relay because it is the most common medium in which this problem occurs. The packet loss can be verified through the interface statistics by looking at the number of packet drops and determining whether that number is constantly incrementing.

Figure 3-47 shows the network setup that can produce RIP route flapping.

Figure 3-47. Network Vulnerable to RIP Route Flapping

Figure 3-48 shows the flowchart to follow to solve this problem.

Figure 3-48. Flowchart to Solving the RIP Route Flapping Problem

Debugs and Verification

In a large RIP network, especially , in a Frame Relay environment, there is a high possibility that RIP updates are lost in the Frame Relay cloud or that the RIP interface dropped the update. Again, the symptoms can be present in any Layer 2 media, but Frame Relay is the focus here. This situation causes RIP to lose a route for a while. If RIP does not receive a route for 180 seconds, the route is put in a holddown for 240 seconds and then is purged. This situation is corrected by itself (and time), but, in some cases, configuration changes can be required. For example, consider the output in Example 3-128, where no RIP update has been received for 2 minutes and 8 seconds. This means that four RIP updates have been missed, and we are 8 seconds into the fifth update.

Example 3-128 Routing Table of the Hub Router Showing That the Last RIP Update Was Received 2:08 Minutes Ago

 Hub#  show ip route rip  R   155.155.0.0/16 [120/1] via 131.108.1.1,  00:02:08,  Serial0 R   166.166.0.0/16 [120/1] via 131.108.1.1,  00:02:08,  Serial0 

Example 3-129 shows that there are a large number of broadcast drops on the interface.

Example 3-129 show interfaces serial 0 Output Reveals a Large Number of Broadcast Drops

 Hub#  show interfaces serial 0  Serial0 is up, line protocol is up Hardware is MK5025 Description: Charlotte Frame Relay Port DLCI 100 MTU 1500 bytes, BW 1024 Kbit, DLY 20000 usec, rely 255/255, load 44/255 Encapsulation FRAME-RELAY, loopback not set, keepalive set (10 sec) LMI enq sent 7940, LMI stat recvd 7937, LMI upd recvd 0, DTE LMI up LMI enq recvd 0, LMI stat sent 0, LMI upd sent 0 LMI DLCI 1023 LMI type is CISCO frame relay DTE Broadcast queue 64/64,  broadcasts sent/dropped 1769202/1849660,  interface         broadcasts 3579215 

Solution

The show interfaces serial 0 command further proves that there is some problem at the interface level. Too many drops are occurring at the interface level. This is the cause of the route flapping. In the case of Frame Relay, the Frame Relay broadcast queue must be tuned . Tuning the Frame Relay broadcast queue is out of the scope of this book; several papers at Cisco's Web site discuss how to tune the Frame Relay broadcast queue.

In a non-Frame Relay situation, the input or output hold queue might need to be increased.

Example 3-130 shows that after fixing the interface drop problem, route flapping disappears.

Example 3-130 Serial Interface Output After Adjusting the Broadcast Queue

 Hub#  show interfaces serial 0  Serial0 is up, line protocol is up Hardware is MK5025 Description: Charlotte Frame Relay Port DLCI 100 MTU 1500 bytes, BW 1024 Kbit, DLY 20000 usec, rely 255/255, load 44/255 Encapsulation FRAME-RELAY, loopback not set, keepalive set (10 sec) LMI enq sent 7940, LMI stat recvd 7937, LMI upd recvd 0, DTE LMI up LMI enq recvd 0, LMI stat sent 0, LMI upd sent 0 LMI DLCI 1023 LMI type is CISCO frame relay DTE Broadcast queue 0/256,  broadcasts sent/dropped 1769202/0,  interface broadcasts         3579215 

In Example 3-131, the show ip routes output displays that the routes are stable in the routing table and that the timers are at a value lower than 30 seconds.

Example 3-131 show ip routes Output Reveals Stable RIP Routes

 Hub#  show ip route rip  R   155.155.0.0/16 [120/1] via 131.108.1.1,  00:00:07,  Serial0 R   166.166.0.0/16 [120/1] via 131.108.1.1,  00:00:07,  Serial0